Continuous production of antibacterial carboxymethyl chitosan-zinc supramolecular hydrogel fiber using a double-syringe injection device

Highlights

• Continuous production of carboxymethyl chitosan-zinc hydrogel fiber is achieved.

• A double-syringe injection device consisting of all commercial parts is reported.

• The CMCh-Zn hydrogel fibers exhibit good mechanical and antibacterial properties.

Abstract

Large-scale production of an antibacterial hydrogel is of critical importance for its practical application in biomedical field. In this regard, herein we report on the construction of a double-syringe injection device by using all the commercial parts and its use for continuous production of carboxymethyl chitosan-zinc (CMCh-Zn) supramolecular hydrogel fiber. The resultant CMCh-Zn hydrogel fibers exhibit good stretchability and knittability. The Zn loading into the hydrogel fibers can be easily controlled by adjusting the concentration of Zn²⁺ solution. Scanning electron microscope measurements indicate that the CMCh-Zn hydrogel fibers have a relatively smooth and thin skin layer, as well as, a 3-dimensional interconnected microporous interior architecture. Antibacterial activities of the CMCh-Zn hydrogel fibers against both Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli are also investigated. The results show that the intrinsic blue fluorescence of the as-prepared CMCh-Zn hydrogel fibers can be employed as optical indicator of their antibacterial effectiveness.

Introduction

Over the past several decades antibacterial hydrogels (ABHs) had received a great deal of research interest because of their superior relevance to numerous biomedical applications, such as wound dressing, drug delivery and tissue engineering [[1], [2], [3], [4]]. Traditional method to prepare ABHs falls into two categories: 1) chemical conjugation or physical loading of antibacterial agents (e.g., antibiotics, silver nanoparticles, bactericidal peptides, etc.) onto a conventional hydrogel network [[5], [6], [7]]; 2) direct cross-linking of certain cationic polymers, which exhibit intrinsic contact-killing property to bacterium [[8], [9], [10]]. However, the synthesis of bactericidal peptides/polyelectrolytes still suffers from low product yield and most conventional synthetic polymers exhibit poor biocompatibility, thus limiting their wide application in biomedical field. Therefore, despite of extensive studies on ABHs, it is still highly desirable to develop a robust and controllable strategy for large-scale production of ABHs with a biocompatible material.

In contrast to synthetic polymers, biopolymers from natural resource usually exhibit good biocompatibility, biodegradability, availability and renewability. Therefore, extensive efforts had been placed on the development of ABHs using biopolymers (e.g., cellulose, alginate, chitosan, etc.) [[11], [12], [13], [14], [15]]. Of particular interest, chitosan and its derivatives exhibit intrinsic antibacterial property due to their polycationic nature, making them potentially useful for numerous biomedical applications [[16], [17], [18], [19]]. Carboxymethyl chitosan (CMCh) is an important derivative of chitosan and can be easily obtained by treating a commercial chitosan with monochloroacetic acid in an alkaline medium [20]. When compared with pristine chitosan, CMCh was reported to have better biocompatibility, biodegradability, as well as enhanced antioxidant activity [[21], [22], [23], [24]]. Consequently, CMCh had been used as biomaterials in various biomedical fields [[25], [26], [27]].

Our group previously reported a facile method for the preparation of CMCh-metal ion (e.g., Ag⁺, Cu²⁺ and Zn²⁺) supramolecular hydrogel by simply mixing a CMCh solution with a metal ion solution at an appropriate pH value, and the resultant hydrogels exhibited excellent antibacterial activity [28]. Considering the fact that topical administration of Zn²⁺ is beneficial for wound healing process [[29], [30], [31]] and have a lower cytotoxicity than Cu²⁺ [32], the CMCh-Zn supramolecular hydrogel is believed to be highly promising as wound dressing material. However, it should be noted that the gelation process of CMCh-Zn hydrogel is very fast (i.e., usually within a few of seconds especially when using a high concentration of Zn²⁺ solution) [28,33], making it difficult to obtain a homogeneous hydrogel on a large scale by simply mixing CMCh with Zn²⁺ solution. Thus, the objective of the current project is to develop a suitable device and strategy for the continuous production of antibacterial CMCh-Zn supramolecular hydrogel.

According to the literature, both microfluidic injection and in-line mixing techniques allow for continuous production of biopolymer-based nano/micro structures, including nanoparticle, microcapsule, as well as, microfiber [[34], [35], [36], [37], [38], [39], [40], [41]]. For instance, alginate supramolecular hydrogel fibers, which were prepared by using different microfluidic devices, had been extensively studied for biomedical applications, such as localized delivery of therapeutics, tissue remedy at specific position and 3D culturing of cells [[42], [43], [44], [45], [46], [47], [48]]. However, most reported microfluidic devices themselves rely on sophisticated fabrication process and thus are not suitable for large-scale production of ABHs.

In order to reduce both the complexity and the cost of a microfluidic device, in this work, a double-syringe injection device was designed and built using a syringe-pump and some Luer-Lok fittings and polypropylene tubings. Then, the flow rates of both CMCh and Zn²⁺ solution were optimized for the continuous production of homogeneous CMCh-Zn hydrogel fiber. The loading of Zn²⁺ into the CMCh-Zn hydrogel fibers was controlled by adjusting the Zn²⁺ concentration, and then determined by inductively coupled plasma atomic emission spectrometer (ICP-AES). Both the surface and inner morphologies of the CMCh-Zn hydrogel fibers were examined by using scanning electron microscope (SEM). At last, antibacterial activities of the CMCh-Zn hydrogel fibers against both Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli) were investigated via inhibition zone method.